Parameters

Specifies the reference value for the stencil test.
ref is clamped to the range
02n-1,
where
n
is the number of bitplanes in the stencil buffer. The
initial value is 0.

mask

Specifies a mask that is ANDed with both the reference value
and the stored stencil value when the test is done. The initial value
is all 1's.

Description

Stenciling,
like depth-buffering,
enables and disables drawing on a per-pixel basis.
Stencil planes are first drawn into using GL drawing primitives, then
geometry and images are rendered using the stencil planes to mask out
portions of the screen.
Stenciling is typically used in multipass rendering algorithms
to achieve special effects,
such as decals,
outlining,
and constructive solid geometry rendering.

The stencil test conditionally eliminates a pixel based on the outcome
of a comparison between the reference value
and the value in the stencil buffer.
To enable and disable the test, call glEnable and glDisable
with argument GL_STENCIL_TEST.
To specify actions based on the outcome of the stencil test, call
glStencilOp or
glStencilOpSeparate.

There can be two separate sets of func, ref, and
mask parameters; one affects back-facing polygons, and the other
affects front-facing polygons as well as other non-polygon primitives.
glStencilFunc sets both front
and back stencil state to the same values. Use glStencilFuncSeparate
to set front and back stencil state to different values.

func is a symbolic constant that determines the stencil comparison function.
It accepts one of eight values,
shown in the following list.
ref is an integer reference value that is used in the stencil comparison.
It is clamped to the range
02n-1,
where
n
is the number of bitplanes in the stencil buffer.
mask is bitwise ANDed with both the reference value
and the stored stencil value,
with the ANDed values participating in the comparison.

If stencil represents the value stored in the corresponding
stencil buffer location,
the following list shows the effect of each comparison function
that can be specified by func.
Only if the comparison succeeds is the pixel passed through
to the next stage in the rasterization process
(see glStencilOp).
All tests treat stencil values as unsigned integers in the range
02n-1,
where
n
is the number of bitplanes in the stencil buffer.

The following values are accepted by func:

GL_NEVER

Always fails.

GL_LESS

Passes if ( ref & mask ) < ( stencil & mask ).

GL_LEQUAL

Passes if ( ref & mask ) <= ( stencil & mask ).

GL_GREATER

Passes if ( ref & mask ) > ( stencil & mask ).

GL_GEQUAL

Passes if ( ref & mask ) >= ( stencil & mask ).

GL_EQUAL

Passes if ( ref & mask ) = ( stencil & mask ).

GL_NOTEQUAL

Passes if ( ref & mask ) != ( stencil & mask ).

GL_ALWAYS

Always passes.

Notes

Initially, the stencil test is disabled.
If there is no stencil buffer,
no stencil modification can occur and it is as if
the stencil test always passes.